Because these works are typically made openly available after submission, most notably in the Tufts Digital Library, it is preferable to either obtain permission from the rights holder or utilize images that do not require obtaining permission for your intended use.
Dissertation, Thesis, or Capstone
Below are some resources that contain images that may not require obtaining permission for your use:
Creative Commons and Public Domain Images
Google Image Search
After keyword searching, click Tools at the top of the results screen. Then under Usage Rights, select Creative Commons licenses. Google will search Flickr, Wikimedia Commons, as well as other creative commons-licensed content.
Health Education Assets Library (HEAL)
This search portal is supported by the Mountain West Digital Library, a collaborative of libraries, museums, and archives.